1. Field of the Invention
This invention relates to the field of underground pipelines and more specifically to a pipeline installation in areas in which the pipeline extends through soil zones having frost heave driving forces.
2. Description of the Prior Art
The design of underground pipelines in regions of discontinuous and continuous permafrost must incorporate the same design parameters for construction in warm climates, such as internal pressure, temperature differentials and soil conditions. Additionally, in cold climates with permafrost conditions, the effects of pipeline differential settlement due to thawing, the complex effects of differential frost heave, and the effects of massive movements such as solifluction, soil creepy and cryoturbation must be included in the design. These parameters become even more acute when any pipeline is designed to transport natural gas at high pressures. High pressures mandate that the pipelines be buried. Generally, in arctic and sub-arctic environments, soils are highly variable in composition, texture, moisture content, and stability under thawed conditions. Thus, the soils are highly heterogeneous with respect to critical parameters that determine the strength and bearing capacity of the soil mass, which in turn constrains the design of the buried pipeline.
Because of these parameters, an optimal design would result in the construction of chilled gas pipelines (operated below zero degrees centigrade) in regions of continuous permafrost and warm pipelines (operated above zero degrees centigrade) in regions with unfrozen soil. However, in regions of discontinuous permafrost the operational design becomes more complex. If the pipeline is operated as a chilled facility in unfrozen and heterogeneous soil, differential frost heaves occur because of the asymmetrical frost bulb that develops with time around the pipe. An example of such an asymmetrical frost bulb is shown FIG. 1, which is derived from FIG. 8 of Thermal effects in permafrost, Gold, L. W., Johnston, G. H., Slusarchuk, W. A., and Goodrich, L. E., 1972, in Proceedings of Canadian Northern Pipeline Research Conference, Conference, Ottawa, 2–4 February, Division of Building Research, National Research Council of Canada. This figure shows that, over time, the ground around the chilled pipeline freezes both horizontally and vertically. Thus, for a pipeline operating at 20° F., after 5 years, the ground around the pipe will freeze to an extent of 20 feet on the horizontal and 25 feet on the vertical. For a pipeline operating at −166° F., the ground will freeze to an extent of 40 feet on the horizontal and almost 60 feet on the vertical after the same five years. The problem is that the development of the frost bulb results in frost heave. With differing soils of differing moisture content and thermal conductivity along the pipeline, differential frost heave develops. Differential frost heave is also produced by asymmetry of the frost bulb. Differential frost heave results in high stress concentrations around the pipe that can result in a catastrophic failure particularly in high pressure pipelines.
Two examples of the types of heaving forces are shown in FIGS. 2 and 3, which are derived from FIG. 7.2 of Construction in cold regions, McFadden, T. T., and Bennet, F. L., 1991: John Wiley and Sons, New York. FIG. 2 shows settlement or driving forces and FIG. 3 shows heaving or driving forces. In either case, the driving force is caused by forces acting on a pipe 100 that is in an area of higher settling soils 120 (FIG. 2) or higher heaving soil 130 (FIG. 3) lying between areas of lesser heaving or settling soils 110.
If the pipeline is operated as a warm facility in areas of frozen soil, the soil will thaw and differential settlement may occur. Extreme conditions occur at the interfaces between ice rich frozen soils and unfrozen soils.
These differential movements produce horizontal and vertical transverse stress, longitudinal stress, and torsional stress in the pipeline. Most of the attempts to solve these problems have addressed the vertical settlement and vertical frost heave induced stresses.
For example, one solution to differential vertical frost heave under a chilled pipeline includes placing insulation under the pipeline, adding electric heating under the insulation, and placing a system of sensors and controls to maintain equilibrium under the insulating pad. This solution has been proposed in Chilled gas pipeline-frost heave design, Svec, O. J., in T. S. Vinson (ed) The Northern Community: A search for a quality environment, American Society of Civil Engineers Specialty Conference, Seattle, Apr. 8–10, 1981, pp. 705–718. This proposed system is complex and requires both power and sensor systems to monitor each site of differing soil conditions (frost heave susceptibility). Moreover, the mathematical model used by Svec for the proposed design assume homogeneous soil conditions and only vertical migration of groundwater to the freezing front around the chilled pipeline. Such assumptions are overly simplistic because discontinuous permafrost soils are not homogeneous and significant lateral movement of groundwater is the norm. The frost bulb developing around a chilled pipeline will be larger on the side of the pipe up the hydraulic gradient than on the side down the gradient. The result is a significant torsional stress on the pipe. The design proposed by Svec also does not address the effects of mass movements on slopes at high angles on the buried pipeline.
Several other methods can be used to stabilize such soils. First, the soil can be excavated over a large enough area to remove the frost susceptible soils. This is an expensive and possibly environmentally problematic solution. Second, techniques can be employed that maintain the soils in a perpetually frozen state. Examples of this technique can be found in U.S. Pat. Nos. 3,650,119 and 4,464,082. U.S. Pat. No. 3,650,119 describes a system that keeps ground permanently frozen when transporting warm oil through a pipeline. Here, the ground is kept frozen by insulating the warm oil pipe and running an uninsulated chilled gas pipeline in the same trench. The chilled gas line keeps the surrounding ground from thawing, keeping both pipes in stable ground. Although it is effective, this method requires the installation of a second pipeline, which adds to the cost. Moreover, it can only be used where a ready supply of chilled gas is available.
U.S. Pat. No. 4,464,082 teaches a case for protecting a chilled gas line from frost heaves. In this case, the upper 300 degrees of pipe is covered with insulation. The bottom 60 degrees of pipe are left uncovered. In this way, the pipe helps keep the ground below it frozen while the active layer above is free to freeze and thaw. In this case, the inventors were concerned about uplift of the pipe caused by frost heaving under the pipe. By keeping the soil below the pipe chilled, they believed that the pipe would not experience such uplifts. However, research shows that this method does not work.